Parts which need lubrication, e.g., bearings, are generally packed with lubricative compositions such as lubricating oils and semisolid greases. However, since these lubricative compositions including semisolid greases, not to mention lubricating oils, become more flowable with increasing temperature during operation of the bearings, the lubricant-filled spaces should be sealed with a sealing member, e.g., a sealing plate, in order to prevent the lubricative compositions from flying out. However, sealing makes the bearings have a complicated structure. In some cases, it is impossible to dispose a sealing member, as in small-sized bearings.
As a lubricative composition eliminating the necessity of a sealing member, a solid lubricative composition obtained by mixing a lubricating oil and a resin has been developed and put to practical use. In this resin-based lubricative composition, the lubricating oil held in the resin oozes out to the resin surface to exert lubricity. For example, polyethylene containing a lubricating oil is described in U.S. Pat. Nos. 3,729,415, 3,547,819, and 3,541,011. In British Patent 1,163,123 is described a similar lubricative composition which employs polypropylene besides polyethylene. Further, another similar lubricative composition employing polymethylpentene is described in U.S. Pat. Nos. 4,239,632 and 4,146,487.
Further, a polyethylene-based lubricative composition employing a grease in place of a lubricating oil is described in JP-A-55-137198. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
In JP-A-55-137198 is described a lubricative composition comprising ultrahigh-molecular polyethylene having a molecular weight of about from 1.times.10.sup.6 to 5.times.10.sup.6 and a lubricating grease held in the polyethylene.
Furthermore, a lubricative composition based on an oil-containing crosslinked resin is described in JP-B-63-21698 which comprises crosslinked polyethylene obtained by heat-treating high-density polyethylene in the presence of a crosslinking agent and, contained in the crosslinked polyethylene, 15 to 45 wt % polyoxyalkylene glycol monoether based on the amount of the whole crosslinked polyethylene. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
However, of the resin-based lubricative compositions described above, the lubricative composition comprising polyethylene and either a lubricating oil or a grease has the following problems. That is, it is unusable at temperatures not lower than 80.degree. C. because the polyethylene, having a low melting point, begins to soften at about 80.degree. C. and completely melts at 130 to 140.degree. C. In addition, there is another drawback that a mixture of polyethylene and either a lubricating oil or a grease should be maintained at a temperature (130-140.degree. C.) not lower than the melting point of the polyethylene for a long time in order to melt the polyethylene to homogeneously incorporate the lubricating oil or grease into the polyethylene and this treatment may deteriorate the lubricating oil and grease.
The lubricative composition based on ultrahigh-molecular polyethylene is unsatisfactory in mechanical strength and moldability although it has improved heat resistance. Hence, when packed into a bearing or the like, the composition suffers a deformation or breakage with the revolution of the rolling elements and comes out from the filled space.
The lubricative composition based on crosslinked polyethylene is free from the problems concerning mechanical strength and moldability which problems are encountered in the above-described composition based on ultrahigh-molecular polyethylene. However, because of the low polarity of polyethylene itself, the polymer shows poor retention of a highly polar lubricating oil. For example, in the case of the polyoxyalkylene glycol monoether mentioned above, the amount thereof which can be retained by the polyethylene is as small as 15 to 45 wt % based on the amount of the whole polyethylene. Hence, when the lubricative composition is applied to a bearing or the like, the oozing of the lubricating oil ends within a short time, posing a problem concerning lubricity life.
Further, the lubricative composition comprising polypropylene and a lubricating oil and that comprising polymethylpentene and a lubricating oil have similar problems. That is, the composition comprising polypropylene and a lubricating oil is unusable at temperatures not lower than 120.degree. C. because the polypropylene, although having a slightly higher melting point than the polyethylene mentioned above, begins to soften at about 120.degree. C. and completely melts at 160 to 170.degree. C. In addition, it is necessary that for homogeneously incorporating a lubricating oil or grease, the polypropylene/lubricant mixture be maintained at a higher temperature (160-170.degree. C.) than the polyethylene composition for a long time and this treatment deteriorates the lubricating oil and grease more severely than that heated with polyethylene.
On the other hand, the lubricative composition comprising polymethylpentene and either a lubricating oil or a grease is usable at high temperatures since the polymethylpentene has a higher melting point than polyethylene, polypropylene, etc. However, it is necessary that for homogeneously incorporating the lubricating oil, the polymethylpentene/lubricant mixture be maintained at an exceedingly high temperature of 230 to 260.degree. C., upon which treatment the lubricating oil or grease deteriorates considerably.
The conventional rolling bearings packed with a lubricant-containing polymer member have had the following problems. In particular, in the case of a bearing in which the whole space between the outer and inner races except the space occupied by the rolling elements is completely filled with a lubricant-containing polymer member, the friction between the lubricant-containing polymer member and the inner wall of the outer race and between the polymer member and the outer wall of the inner race may cause a torque increase and a bearing temperature increase. Further, a-friction-generated powder of the lubricant-containing polymer member inhibits the smooth revolution of each revolving member to cause the so-called gripping phenomenon, vibration of the whole bearing, etc., and as a result, the bearing comes to no longer perform its function.
It should be further noted that the disposition of a lubricant-containing polymer member in a rolling bearing is accomplished by packing the lubricant-containing polymer in a gel or paste state into the bearing at the space formed by the outer and inner races and the rolling elements, heating the packed polymer, and then cooling the polymer to solidify it to thereby hold the polymer member in the space. Therefore, when some kinds of polymers, in particular ones which contract little according to the solidification or ones which are too elastic, are used, there are cases where the outer and inner races and the rolling elements are pressed by the lubricant-containing polymer member and the bearing comes not to work at all.